1. Field of the Invention
The field of this invention relates to the liquid-phase oxidation of an aromatic compound having oxidizable substituents to the corresponding aromatic polycarboxylic acid, and more particularly concerns the oxidation of an aromatic compound having at least three oxidizable substituents.
2. Description of the Prior Art
Heretofore, it has not been possible generally to achieve high yields of the resulting aromatic polycarboxylic acid when a aromatic compound having three or more oxidizable substituents are oxidized in the liquid phase and in a solvent to its corresponding aromatic polycarboxylic acid in the presence of a catalyst having cobalt, manganese and bromine components. This problem is particularly severe when an aromatic compound having four or more oxidizable substituents is being oxidized.
The reasons for this problem are not clear, but an aromatic polycarboxylic acid produced when three or more oxidizable substituents, especially on the same aromatic ring, are oxidized has a propensity to precipitate the metal component(s) of the aforesaid catalyst from the reaction mixture. The aforesaid catalyst is also deactivated when an aromatic polycarboxylic acid having two or more carboxylic groups ortho to each other on the aromatic ring is formed. For example, when durene is oxidized to pyromellitic acid, the deactivation of the aforesaid catalyst occurs when the oxidation of durene is about fifty percent complete. For an aromatic polycarboxylic acid having more than two carboxylic acid groups ortho to each other on the aromatic ring, deactivation of the aforesaid catalyst occurs when the oxidation reaction is less than fifty percent complete.